1. Field of the Invention
The present invention relates to a signal processing method and the circuit thereof, especially to a method and a circuit for equalizing and compensating IQ imbalance simultaneously.
2. Description of the Prior Art
In communication systems, a carrier is frequently utilized to carry baseband signals that contain data. Generally, a carrier is a high frequency signal. After receiving a radio frequency signal, a receiver initially down converts the radio frequency for the convenience of further process. Recently, due to manufacturing progress and consideration of cost reduction issues, more and more RF circuits in the receiving end have adopted direct down conversion to directly down convert the radio frequency signals into baseband signals. In general, a transmitter adopts a modulation scheme with high bandwidth efficiency because of the bandwidth limitation. A quadrature amplitude modulation (QAM) is a frequently utilized modulation scheme, especially in the cases where a high-resolution digital television signal is transmitted. In such a case, a 256 QAM modulation scheme is often adopted.
When the front end of a receiving terminal adopts an RF front end that utilizes direct down conversion, an IQ imbalance issue is frequently introduced. The IQ imbalance results in the interfering of the function of a QAM receiver. As a result, the receiving end can be equipped with an IQ imbalance compensation circuit for compensating the received radio frequency signal that possesses an IQ imbalance problem caused by direct down conversion. In addition, a channel is generally accompanied by a multi-path issue in the transmission process, so the receiving end requires an equalizer to solve the problem caused by multi-path effects during signal transmission.
Please refer to FIG. 1. FIG. 1 shows a block diagram of a prior art receiver 100. The receiver 100 contains a direct down converter 110, an IQ imbalance compensating circuit 120, and an equalizer 130. After the incoming signal S1 is received by the direct down converter 110, the incoming signal S1 is transmitted in two different paths. Ideally, in the two paths, the incoming signal S1 is multiplied by g sin wt and g cos wt respectively by a mixer 116 and a mixer 118 (where g is the gain, w is the angular frequency). However, the phases of these two signals, g sin wt and g cos wt, are probably not orthogonal, and the gains of these two signals may also be different. In other words, the oscillating signals utilized by the direct down converter 110 are probably (g+α) sin wt and g cos(wt+θ), where α is the gain imbalance and θ is the phase imbalance. As a result, the two signals S1_I and S1_Q, which are respectively filtered by the LPF's 112 and 114, have an IQ imbalance issue. Typically, the IQ imbalance compensating circuit 120 generates a compensation coefficient through a calibration method for compensating the direct down converter 110. Once the calibration process of the IQ imbalance compensating circuit 120 has been achieved, the IQ imbalance compensating circuit 120 utilizes the same compensation coefficient to compensate all signals under all kinds of operating environments.
The compensated signals S1_I′ and S1_Q′ enter the equalizer 130 and are therefore equalized. In general, assuming that the IQ imbalance does not exist, a signal (T) transmitted by a transmitter, passing through a channel (H), and being received by a receiver (R), the transmitting signal T and the receiving signal R are therefore expressed by the following equation:R(n)= H(n)× T(n)=( Hi(n)+j Hq(n))×( Ti(n)+j Tq(n)) nε1, 2, 3, . . .  Eq. (1),
where H represents a channel model. The above equation can also be expressed in the matrix form:
                                          [                                                            R                  i                                ⁡                                  (                  n                  )                                                                              R                  q                                ⁡                                  (                  n                  )                                                      ]                    =                                    [                                                                                          H                      _                                        i                                                                              H                      _                                        q                                                  ⁢                                                      -                                                                  H                        _                                            q                                                                                                  H                      _                                        i                                                              ]                        ⁡                          [                                                                    T                    _                                    i                                                                      T                    _                                    q                                            ]                                      ,                            Eq        .                                  ⁢                  (          2          )                    
It is obvious that the two elements on the two respective diagonals of the channel model have a certain relation: the two elements on the main diagonal are the same, and the signs of the two elements on the other diagonal are opposite. Generally, the channel model H is not known in advance, so the equalizer 130 executes an adaptive algorithm to find the adaptive form of the channel model H. One frequently utilized adaptive algorithm is the Least-Mean-Square Algorithm. (Please refer to “Least-Mean-Square Adaptive Filters”, Ch. 5 of “Adaptive Filter Theory”, by Simon Haykin, 4th Ed., 2004, ISBN: 0-1304-8434-2.) As a result, the adaptive algorithm can be expressed as:
                                          [                                                                                w                    _                                    i                                                                      w                    _                                    q                                            ⁢                                                -                                                            w                      _                                        q                                                                                        w                    _                                    i                                                      ]                    =                                    [                                                                                          H                      _                                        i                                                                              H                      _                                        q                                                  ⁢                                                      -                                                                  H                        _                                            q                                                                                                  H                      _                                        i                                                              ]                                      -              1                                      ,                            Eq        .                                  ⁢                  (          3          )                    
where
                              w          _                i            ⁡              (                  n          +          1                )              =                                        w            _                    i                ⁡                  (          n          )                    +              u        ·                  (                                                                      e                  i                                ⁡                                  (                  n                  )                                            ·                                                                    R                    _                                    i                                ⁡                                  (                  n                  )                                                      +                                                            e                  q                                ⁡                                  (                  n                  )                                            ·                                                                    R                    _                                    q                                ⁡                                  (                  n                  )                                                              )                                                              w            _                    q                ⁡                  (                      n            +            1                    )                    =                                                  w              _                        q                    ⁡                      (            n            )                          +                  u          ·                      (                                                                                e                    q                                    ⁡                                      (                    n                    )                                                  ·                                                                            R                      _                                        i                                    ⁡                                      (                    n                    )                                                              +                                                                    e                    i                                    ⁡                                      (                    n                    )                                                  ·                                                                            R                      _                                        q                                    ⁡                                      (                    n                    )                                                                        )                                ,  u being the step-size, ei and eq being errors, and Ri and Rq being data.
Similarly, in the adaptive matrix w, the two elements on the main diagonal are the same, and the signs of the two elements on the other diagonal are opposite.
The equalizer 130 takes the adaptive matrix was its equalization coefficient to equalize the signals S1_I′ and S1_Q′. The required signals S1_I″ and S1_Q″ are then generated. Therefore, the equalizer 130 solves the multi-path problem occurring from signals passing through the channel.
Consequently, after the down-converted signals S1_I and S1_Q are processed by the IQ imbalance compensating circuit 120 and the equalizer 130, the IQ imbalance caused by the direct down converter 110 is compensated and the multi-path issue caused by the channel is also solved. However, as mentioned above, the IQ imbalance compensating circuit 120 utilizes a calibration method whose characteristic is that the IQ imbalance compensating circuit 120 calibrates only one time according to a specific frequency, and afterwards the IQ imbalance compensating circuit 120 utilizes the same compensation coefficient to compensate the signals of all kinds of frequencies in all operational conditions. In practical operation, a change in temperature may directly affect the direct down converter 110, so the IQ imbalance changes due to the change in temperature. In addition, the IQ imbalance also changes with respect to signals of different frequencies. As a result, a compensation circuit is required to compensate IQ imbalance of signals of different frequencies in all kinds of changes in the operational conditions, such as a temperature change.